The present invention relates to the mounting of electronic modules upon printed circuit boards and, more particularly, to a structure and method for mechanically and electrically connecting the module to the printed circuit board through a plurality of compliant electrical conductors attached to contact surfaces, said connection having an improved fatigue life.
In many computer and other electronic circuit structures, an electronic module such as a Central Processor Unit (CPU), memory module or ASIC, must be connected to a printed circuit board (hereinafter sometimes xe2x80x9cPCBxe2x80x9d). In connecting a module to a PCB, a plurality of individual electrical contacts on the base of the module must be connected to a plurality of corresponding individual electrical contacts on the PCB. This set of contacts on the PCB dedicated to receiving the module contacts is known as a land grid array (hereinafter sometimes xe2x80x9cLGAxe2x80x9d) site.
The required interconnect density on today""s printed circuit boards and associated module(s) are such that distances between contacts within an LGA site as small as 0.8 millimeter must be supported. In order to connect a module structurally and electrically to an LGA site on a PCB in a reliable fashion, a number of problems must be overcome. First, the initial alignment of the respective contacts on the module and the PCB with designated conductive connections must be achieved. Second, a reliable, electrically conductive connector with a satisfactory fatigue life and stable contact resistance for connecting module and PCB contacts must be provided. And, lastly, where the conductive connector is permanently attached to one or both of the module and PCB contacts with solder or some other permanent adhesive compound, the adhesive or solder may xe2x80x9cwickxe2x80x9d through the connector array and spread in an undesirable fashion, perhaps even creating electrical xe2x80x9cshortsxe2x80x9d between adjacent connectors or contacts. What is also needed is a way to prevent the permanent adhesive or solder from xe2x80x9cwickingxe2x80x9d through the connector and spreading beyond the desired attachment interface.
With respect to the first problem, each contact within a land grid array site on a circuit board must be aligned with an individual electrically conductive connector for electrically connecting the contact to a corresponding contact on a module. Each individual connector must also be aligned with its designated module contact. It is also important that the individual connectors are electrically isolated from each other, in order to prevent undesired cross-connections, such as a xe2x80x9cshort-circuitxe2x80x9d connection between adjacent PCB and module contacts.
With respect to the second problem, the connectors must provide a reliable connection between PCB and module contacts that has an acceptable performance relating to stable contact resistance and fatigue life. Accordingly, the connector must resist forces that effectively work to separate the connectors from the PCB and module contacts. Such forces are inherently generated by the use of a module/PCB assembly.
For example, module substrates are typically fabricated from ceramic materials. The coefficient of thermal expansion (hereinafter sometimes xe2x80x9cCTExe2x80x9d) of ceramic modules typically ranges from 2 to 10 parts-per-million (ppm). This is much lower than that of a PCB fabricated from an epoxy resin/glass cloth substrate, which will typically have a CTE in the range of about 15 through about 20 ppm. Therefore, an assembly of a ceramic substrate module and an epoxy resin substrate PCB will be an assembly of xe2x80x9cthermally mismatchedxe2x80x9d components with respect to their CTE""s. During the operation of this assembly in a typical computing application, the assembly will be subject to heating and cooling cycles inherent in the electrical and mechanical use of the assembly. Since the CTE""s of the module and the PCB do not correspond, they will expand and contract at different rates as the assembly is subjected to heating and cooling cycles. Since they will expand and contract at different rates, the ceramic module and epoxy resin PCB will necessarily move with respect to each other during heating and cooling cycles, resulting in shear forces acting upon module and PCB contact connections. The connectors located at the corners of the module have the highest amount of shear strain, because they are the farthest from the neutral point at the center of the module; they have the largest distance to neutral point (DNP) value and, therefore, must withstand the largest displacement force during the heating and cooling cycles of the structure. The average strain imposed upon an individual contact connection is quantified by dividing the relative in-plane displacement between the module contact and the PCB contact by the height of the contact connection, also defined as the deformable length of the contact.
Vibration forces also act upon the electrical contacts. Any connection between module and PCB contacts must reliably withstand vibration forces inherent in the operation of the assembly such as, for example, the mechanical vibrations generated by cooling fans and other mechanical equipment. The connection must also withstand mechanical vibration forces, which arise from physical handling of the assembly during manufacturing, and from handling and movement of the device that the assembly may be installed into.
A typical prior art means of connecting the module contacts to the PCB contacts is to use solder hierarchy. For example, a high-melt solder is applied as a ball or columnar shape to each of the contacts in a chip to module substrate contact array and reflowed. A low-melt solder paste is applied to each of the contacts within a corresponding PCB contact array to module substrate. The two contact arrays are aligned and brought into contact with each other to form an assembly. The assembly is heated in order to reflow the low-melt paste, and the assembly is then allowed to cool and thereby solidify the low-melt solder interface into a semi-rigid permanent connection between the PCB and module contacts within the aligned arrays without disturbing the high-melt solder joints.
Since the average shear strain imposed upon an individual corner contact connection is determined by dividing the differential Distance to Neutral Point (DNP) displacement by the deformable length of the contact, a typical solder ball contact connection, which is about 0.03 inches in height, is less preferred than a solder column contact connection, which is typically about 0.05 to about 0.08 inches high. Solder columns, accordingly, afford an increased thermo-mechanical fatigue life in comparison to solder ball connections, typically by a factor of about 2 to about 3. However, solder column technologies are more difficult to form due to cast in place or clasp and attach processes. Furthermore, since solder connections deform inelastically and in response to thermal mismatch shear displacements, multiple heating and cooling cycles can eventually cause solder connections to fail.
Moreover, as the array density of the connectors increases, the application of solder to the contacts is problematic in preventing the solder from spreading outside of the contact areas and causing undesirable shorts between adjacent contacts.
Therefore, what is needed is a method and structure for aligning the PCB and module contacts. What is also needed is a method and structure that provide a reliable electrical connection between the module and PCB contacts that withstands forces from vibration and accommodates shear displacements from mismatches in coefficients of thermal expansion between the module components and the PCB substrate components. And, lastly, it is also preferred that the method and structure prevent the undesirable spread of solder or other conductive adhesives beyond the desired contact/connector interface.
A method and structure for electrically and mechanically interconnecting an array of printed circuit board contacts to an array of module contacts with a plurality of deformable resilient electrical conductors. A portion of the conductors is rigidly affixed to at least one of the contact arrays. Another portion of the conductor may deform longitudinally and laterally responsive to movement of the printed circuit board relative to the module, while maintaining the electrical connection of the contact arrays. An interposer with apertures extending through the interposer carries the conductors in the apertures and is used to align the conductors with the contacts. A method for excluding a rigid adhesive means from a portion of the resilient conductor is also taught.